The invention pertains to a transmitting output stage with adjustable output power and to a process for amplifying a signal in a transmitting output stage.
In modern mobile communications systems, an especially long so-called “stand-by” time is desirable. This term is used for the time during which a mobile communications unit can remain in an active or ready-to-receive state without the need to recharge the battery required for operation. This can be contrasted with the “talk” time, which is the length of time during which continuous transmission or reception is can proceed in the mobile communications device. Customer demand for increasing stand-by and talk times requires not only new types of batteries with higher capacity but also the most efficient possible reduction of the power consumption of the mobile communications device in its various operating modes.
One possibility includes reducing the transmitting power of the mobile communications device, because this has the effect of reducing the power consumption of the amplifier, which makes a major contribution to overall power consumption. Reducing the transmitting power is also a logical option especially when the mobile communications device is located near a base station, because in such cases a low output power is sufficient for satisfactory signal reception. As a result, the current uptake and thus the power consumption can be lowered. As provided in the mobile radio standard UMTS, for example, the base station can instruct the mobile communications device to increase or to decrease its transmitting power.
In modern mobile radio standards such as WCDMA/UMTS, WLAN, or IS95, the data to be transmitted are modulated with respect to both the amplitude and the phase of the signal. As a result, the input amplitude in a power amplifier of the mobile communications device changes over time. The average output power of the mobile communications device is adjusted in turn by regulation of the power amplifier.
FIG. 1 shows a probability density function for a mobile radio standard of this type. It is easy to see that the probability for the output power of a typical high-frequency signal is in the area of 10 dBm. Nevertheless, there is also a relatively high probability for very high output powers of +20 dBm to +30 dBm. As a result, the amplifiers in the transmission path typically fulfill the requirement of having a sufficiently linear transmission behavior over the entire output power range and especially at these high power levels. Otherwise, there will be distortions in the amplitude and in the phase of the signal during the transmission of signals at high output power levels. As a result, transmission errors can be caused, and the data error rate will increase sharply.
To guarantee that the mobile communications device will also provide sufficient transmitting power even when the base station is a long distance away, the output stage of the transmission path and the high-frequency power amplifiers in the output stage are designed for the maximum case which can occur. The power amplifiers in the output stage, however, cannot be equally efficient for every possible input power. FIG. 2 shows a typical curve of the output power as a function of the input power in comparison with the curve of the overall efficiency of the power amplifier versus the input power (PAE, Power Added Efficiency). The efficiency is obtained by dividing the difference between the output power and the input power by the overall power consumption of the amplifier.
It can be seen from the two curves that, at low input power, the efficiency of the power amplifier is not linear; instead, it decreases much more quickly than linear behavior would demand and almost disappears around an input power of −20 dBm. The reason for this is that, at low input levels, the efficiency is determined primarily by the idling current which flows through the power amplifiers in the transmitting output stage. These currents can be decreased only in a certain range, because otherwise the amplifiers will not have a linear characteristic and thus will no longer provide proportional amplification.
There are various approaches which can be taken to solve this problem. They include the one described by J. Staudinger in “Applying Switched Gain Stage Concepts to Improve Efficiency and Linearity for Mobile CDMA Power Amplification” (Microwave Journal, September 2000), in which the output stage is bypassed by a bypass line so that a multi-stage amplifier coming right after a driver stage can be connected to the output of the transmitting output stage.